YALE (US) / U. SHEFFIELD (UK) — The most comprehensive family tree for birds yet connects all living bird species—nearly 10,000 in total—and reveals surprising new details about their origins.

Analysis of the family tree shows when and where birds diversified—and that birds’ diversification rate has increased over the last 50 million years, challenging the conventional wisdom of biodiversity experts.

“It’s the first time that we have—for such a large group of species and with such a high degree of confidence—the full global picture of diversification in time and space,” says biologist Walter Jetz of Yale University, lead author of the team’s research paper, published today online in the journal Nature.

“We used fossils and genetic data to estimate the ages of all the different branches of the bird tree so that we could assess how diversity has accumulated through time,” says Gavin Thomas. See the full diagram. (Credit: U. Sheffield)

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“The research highlights how heterogeneously fast diversifying species groups are distributed throughout the family tree and over geographic space,” continues Jetz.

“Many parts of the globe have seen a variety of species groups diversify rapidly and recently. All this leads to a diversification rate in birds that has been increasing over the past 50 million years.”

The researchers relied heavily on fossil and DNA data, combining them with geographical information to produce the exhaustive family tree, which includes 9,993 species known to be alive now.

“This ‘phylogeny’ is important because it is the first that includes all living birds. It means we can ask questions about biodiversity and evolution on a global scale and gain new insight into how diversity has changed over millions of years as well as understand those changes,” says Gavin Thomas of the University of Sheffield.

Filling each niche

The researchers attribute the growing rate of avian diversity to an abundance of group-specific adaptations. They hypothesize that the evolution of physical or behavioral innovations in certain groups, combined with the opening of new habitats, has enabled repeated bursts of diversification.

“The current zeitgeist in biodiversity science is that the world can fill up quickly,” says biologist and co-author Arne Mooers of Simon Fraser University in Canada.

“A new distinctive group, like bumblebees or tunafish, first evolves, and, if conditions are right, it quickly radiates to produce a large number of species. These species fill up all the available niches, and then there is nowhere to go.

“Extinction catches up, and things begin to slow down or stall. For birds the pattern is the opposite: Speciation is actually speeding up, not slowing down.”

Another likely factor has been birds’ exceptional mobility, researchers say, which time and again has allowed them to colonize new regions and exploit novel ecological opportunities.

The researchers were surprised to find that North America and Eurasia and southern South America had experienced especially intense diversification among bird species. (Credit: Matt/Flickr)

Hot spots

In their analysis, the researchers also expose significant geographic differences in diversification rates. They are higher in the Western Hemisphere than in the Eastern, and higher on islands than mainlands. But surprisingly, they say, there is little difference in rates between the tropics and high latitudes. Regions of especially intense recent diversification include northern North American and Eurasia and southern South America.

“This was one of the big surprises,” Jetz says. “For a long time biologists have thought that the vast diversity of tropical species must at least partly be due to greater rates of net species production there.

“For birds we find no support for this, and groups with fast and slow diversification appear to occur there as much as in the high latitudes. Instead, the answer may lie in the tropics’ older age, leading to a greater accumulation of species over time. Global phylogenies like ours will allow further tests of this and other basic hypotheses about life on Earth.”

How much history?

“More widely, one way in which the phylogeny can be used, and which may not be obvious, is in helping to prioritize conservation efforts,” adds Thomas.

“We can identify where species at greatest risk of extinction are on the tree and ask how much distinct evolutionary history they represent. Some species have many close relatives and represent a small amount of distinct evolutionary history whereas others have few close relatives and their loss would represent the disappearance of vast amounts of evolutionary history that could never be recovered.

“Environmental change has very likely affected diversification over time. Climate change could be a part of that through its effects on the extent of different types of habitat.”

Other authors of the study are J.B. Joy of Simon Fraser University in Canada and K. Hartmann of the University of Tasmania in Australia.

The National Science Foundation, NASA, the Natural Environment Research Council (UK), the Natural Sciences and Engineering Research Council of Canada, Simon Fraser University, and the Yale Institute of Biospheric Studies supported the study.